X11 forwarding does not work if su to another user

I have a Red Hat 7.3 server running XVnc. On a Windows 10 desktop I have Putty and Xming installed. Putty is configured for X11 forwarding. When I SSH in as my standard/non-privileged user and launch an X application, it displays on my Windows 10 desktop without issue. Now within same session, if I su to a more privileged account and try to run an X application, it fails with "error: can't open display". In my standard user session if I echo $DISPLAY it is automatically set for me as "IP_ADDRESS:10.0". Under my su session, $DISPLAY is null. I tried exporting the DISPLAY variable to the same value but it now a different error appears: "PuTTY X11 proxy: Unsupported authorization protocol Error: Can't open display:server_IP:10.0". How can I configure the X11 forwarding to work under the context of the other user?